/*
 *  Copyright (C) [2024] smartboot [zhengjunweimail@163.com]
 *
 *  企业用户未经smartboot组织特别许可，需遵循AGPL-3.0开源协议合理合法使用本项目。
 *
 *   Enterprise users are required to use this project reasonably
 *   and legally in accordance with the AGPL-3.0 open source agreement
 *  without special permission from the smartboot organization.
 */

package tech.smartboot.feat.core.common.codec.h2.hpack;


import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

import static tech.smartboot.feat.core.common.codec.h2.hpack.HPACK.bytesForBits;


/**
 * @author 三刀 zhengjunweimail@163.com
 * @version v1.0.0
 */
public final class QuickHuffman {

    /*
     * Mapping of characters to their code information. Code information
     * consists of a code value and a code length. Both components are packed in
     * a single long as follows:
     *
     *     MSB                             LSB
     *     +----------------+----------------+
     *     |~code           |         length~|
     *     +----------------+----------------+
     *     |<----- 32 ----->|<----- 32 ----->|
     *     |<------------- 64 -------------->|
     *
     * The leftmost 32 bits hold the code value. This value is aligned left (or
     * MSB). The rightmost 32 bits hold the code length. This length is aligned
     * right (or LSB). Such structure is possible thanks to codes being up to 30
     * bits long and their lengths being up to 5 bits long (length = 5..30).
     * This allows both components to fit into long and, thus, better locality.
     *
     * Input strings never contain EOS. Thus there's no need to provide EOS
     * mapping. Hence, the length of the array is 256, not 257.
     */
    private static final long[] codes = new long[256];

    private static long codeValueOf(char c) {
        return codes[c] & 0xffffffff00000000L;
    }

    private static long codeLengthOf(char c) {
        return codes[c] & 0x00000000ffffffffL;
    }

    private static final int EOS_LENGTH = 30;
    private static final int EOS_LSB = 0x3fffffff;
    // EOS_LSB is casted to long before the shift, to allow long shift (6 bits)
    // instead of int shift (5 bits):
    private static final long EOS_MSB = ((long) EOS_LSB) << (64 - EOS_LENGTH);

    /*
     * Huffman trie.
     *
     * The root node leads to 257 descendant leaf nodes, of which 256 nodes
     * correspond to characters and 1 node correspond to EOS.
     */
    private static final Node root = buildTrie();

    private static Node buildTrie() {
        TemporaryNode tmpRoot = new TemporaryNode();
        addChar(tmpRoot, 0, 0x1ff8, 13);
        addChar(tmpRoot, 1, 0x7fffd8, 23);
        addChar(tmpRoot, 2, 0xfffffe2, 28);
        addChar(tmpRoot, 3, 0xfffffe3, 28);
        addChar(tmpRoot, 4, 0xfffffe4, 28);
        addChar(tmpRoot, 5, 0xfffffe5, 28);
        addChar(tmpRoot, 6, 0xfffffe6, 28);
        addChar(tmpRoot, 7, 0xfffffe7, 28);
        addChar(tmpRoot, 8, 0xfffffe8, 28);
        addChar(tmpRoot, 9, 0xffffea, 24);
        addChar(tmpRoot, 10, 0x3ffffffc, 30);
        addChar(tmpRoot, 11, 0xfffffe9, 28);
        addChar(tmpRoot, 12, 0xfffffea, 28);
        addChar(tmpRoot, 13, 0x3ffffffd, 30);
        addChar(tmpRoot, 14, 0xfffffeb, 28);
        addChar(tmpRoot, 15, 0xfffffec, 28);
        addChar(tmpRoot, 16, 0xfffffed, 28);
        addChar(tmpRoot, 17, 0xfffffee, 28);
        addChar(tmpRoot, 18, 0xfffffef, 28);
        addChar(tmpRoot, 19, 0xffffff0, 28);
        addChar(tmpRoot, 20, 0xffffff1, 28);
        addChar(tmpRoot, 21, 0xffffff2, 28);
        addChar(tmpRoot, 22, 0x3ffffffe, 30);
        addChar(tmpRoot, 23, 0xffffff3, 28);
        addChar(tmpRoot, 24, 0xffffff4, 28);
        addChar(tmpRoot, 25, 0xffffff5, 28);
        addChar(tmpRoot, 26, 0xffffff6, 28);
        addChar(tmpRoot, 27, 0xffffff7, 28);
        addChar(tmpRoot, 28, 0xffffff8, 28);
        addChar(tmpRoot, 29, 0xffffff9, 28);
        addChar(tmpRoot, 30, 0xffffffa, 28);
        addChar(tmpRoot, 31, 0xffffffb, 28);
        addChar(tmpRoot, 32, 0x14, 6);
        addChar(tmpRoot, 33, 0x3f8, 10);
        addChar(tmpRoot, 34, 0x3f9, 10);
        addChar(tmpRoot, 35, 0xffa, 12);
        addChar(tmpRoot, 36, 0x1ff9, 13);
        addChar(tmpRoot, 37, 0x15, 6);
        addChar(tmpRoot, 38, 0xf8, 8);
        addChar(tmpRoot, 39, 0x7fa, 11);
        addChar(tmpRoot, 40, 0x3fa, 10);
        addChar(tmpRoot, 41, 0x3fb, 10);
        addChar(tmpRoot, 42, 0xf9, 8);
        addChar(tmpRoot, 43, 0x7fb, 11);
        addChar(tmpRoot, 44, 0xfa, 8);
        addChar(tmpRoot, 45, 0x16, 6);
        addChar(tmpRoot, 46, 0x17, 6);
        addChar(tmpRoot, 47, 0x18, 6);
        addChar(tmpRoot, 48, 0x0, 5);
        addChar(tmpRoot, 49, 0x1, 5);
        addChar(tmpRoot, 50, 0x2, 5);
        addChar(tmpRoot, 51, 0x19, 6);
        addChar(tmpRoot, 52, 0x1a, 6);
        addChar(tmpRoot, 53, 0x1b, 6);
        addChar(tmpRoot, 54, 0x1c, 6);
        addChar(tmpRoot, 55, 0x1d, 6);
        addChar(tmpRoot, 56, 0x1e, 6);
        addChar(tmpRoot, 57, 0x1f, 6);
        addChar(tmpRoot, 58, 0x5c, 7);
        addChar(tmpRoot, 59, 0xfb, 8);
        addChar(tmpRoot, 60, 0x7ffc, 15);
        addChar(tmpRoot, 61, 0x20, 6);
        addChar(tmpRoot, 62, 0xffb, 12);
        addChar(tmpRoot, 63, 0x3fc, 10);
        addChar(tmpRoot, 64, 0x1ffa, 13);
        addChar(tmpRoot, 65, 0x21, 6);
        addChar(tmpRoot, 66, 0x5d, 7);
        addChar(tmpRoot, 67, 0x5e, 7);
        addChar(tmpRoot, 68, 0x5f, 7);
        addChar(tmpRoot, 69, 0x60, 7);
        addChar(tmpRoot, 70, 0x61, 7);
        addChar(tmpRoot, 71, 0x62, 7);
        addChar(tmpRoot, 72, 0x63, 7);
        addChar(tmpRoot, 73, 0x64, 7);
        addChar(tmpRoot, 74, 0x65, 7);
        addChar(tmpRoot, 75, 0x66, 7);
        addChar(tmpRoot, 76, 0x67, 7);
        addChar(tmpRoot, 77, 0x68, 7);
        addChar(tmpRoot, 78, 0x69, 7);
        addChar(tmpRoot, 79, 0x6a, 7);
        addChar(tmpRoot, 80, 0x6b, 7);
        addChar(tmpRoot, 81, 0x6c, 7);
        addChar(tmpRoot, 82, 0x6d, 7);
        addChar(tmpRoot, 83, 0x6e, 7);
        addChar(tmpRoot, 84, 0x6f, 7);
        addChar(tmpRoot, 85, 0x70, 7);
        addChar(tmpRoot, 86, 0x71, 7);
        addChar(tmpRoot, 87, 0x72, 7);
        addChar(tmpRoot, 88, 0xfc, 8);
        addChar(tmpRoot, 89, 0x73, 7);
        addChar(tmpRoot, 90, 0xfd, 8);
        addChar(tmpRoot, 91, 0x1ffb, 13);
        addChar(tmpRoot, 92, 0x7fff0, 19);
        addChar(tmpRoot, 93, 0x1ffc, 13);
        addChar(tmpRoot, 94, 0x3ffc, 14);
        addChar(tmpRoot, 95, 0x22, 6);
        addChar(tmpRoot, 96, 0x7ffd, 15);
        addChar(tmpRoot, 97, 0x3, 5);
        addChar(tmpRoot, 98, 0x23, 6);
        addChar(tmpRoot, 99, 0x4, 5);
        addChar(tmpRoot, 100, 0x24, 6);
        addChar(tmpRoot, 101, 0x5, 5);
        addChar(tmpRoot, 102, 0x25, 6);
        addChar(tmpRoot, 103, 0x26, 6);
        addChar(tmpRoot, 104, 0x27, 6);
        addChar(tmpRoot, 105, 0x6, 5);
        addChar(tmpRoot, 106, 0x74, 7);
        addChar(tmpRoot, 107, 0x75, 7);
        addChar(tmpRoot, 108, 0x28, 6);
        addChar(tmpRoot, 109, 0x29, 6);
        addChar(tmpRoot, 110, 0x2a, 6);
        addChar(tmpRoot, 111, 0x7, 5);
        addChar(tmpRoot, 112, 0x2b, 6);
        addChar(tmpRoot, 113, 0x76, 7);
        addChar(tmpRoot, 114, 0x2c, 6);
        addChar(tmpRoot, 115, 0x8, 5);
        addChar(tmpRoot, 116, 0x9, 5);
        addChar(tmpRoot, 117, 0x2d, 6);
        addChar(tmpRoot, 118, 0x77, 7);
        addChar(tmpRoot, 119, 0x78, 7);
        addChar(tmpRoot, 120, 0x79, 7);
        addChar(tmpRoot, 121, 0x7a, 7);
        addChar(tmpRoot, 122, 0x7b, 7);
        addChar(tmpRoot, 123, 0x7ffe, 15);
        addChar(tmpRoot, 124, 0x7fc, 11);
        addChar(tmpRoot, 125, 0x3ffd, 14);
        addChar(tmpRoot, 126, 0x1ffd, 13);
        addChar(tmpRoot, 127, 0xffffffc, 28);
        addChar(tmpRoot, 128, 0xfffe6, 20);
        addChar(tmpRoot, 129, 0x3fffd2, 22);
        addChar(tmpRoot, 130, 0xfffe7, 20);
        addChar(tmpRoot, 131, 0xfffe8, 20);
        addChar(tmpRoot, 132, 0x3fffd3, 22);
        addChar(tmpRoot, 133, 0x3fffd4, 22);
        addChar(tmpRoot, 134, 0x3fffd5, 22);
        addChar(tmpRoot, 135, 0x7fffd9, 23);
        addChar(tmpRoot, 136, 0x3fffd6, 22);
        addChar(tmpRoot, 137, 0x7fffda, 23);
        addChar(tmpRoot, 138, 0x7fffdb, 23);
        addChar(tmpRoot, 139, 0x7fffdc, 23);
        addChar(tmpRoot, 140, 0x7fffdd, 23);
        addChar(tmpRoot, 141, 0x7fffde, 23);
        addChar(tmpRoot, 142, 0xffffeb, 24);
        addChar(tmpRoot, 143, 0x7fffdf, 23);
        addChar(tmpRoot, 144, 0xffffec, 24);
        addChar(tmpRoot, 145, 0xffffed, 24);
        addChar(tmpRoot, 146, 0x3fffd7, 22);
        addChar(tmpRoot, 147, 0x7fffe0, 23);
        addChar(tmpRoot, 148, 0xffffee, 24);
        addChar(tmpRoot, 149, 0x7fffe1, 23);
        addChar(tmpRoot, 150, 0x7fffe2, 23);
        addChar(tmpRoot, 151, 0x7fffe3, 23);
        addChar(tmpRoot, 152, 0x7fffe4, 23);
        addChar(tmpRoot, 153, 0x1fffdc, 21);
        addChar(tmpRoot, 154, 0x3fffd8, 22);
        addChar(tmpRoot, 155, 0x7fffe5, 23);
        addChar(tmpRoot, 156, 0x3fffd9, 22);
        addChar(tmpRoot, 157, 0x7fffe6, 23);
        addChar(tmpRoot, 158, 0x7fffe7, 23);
        addChar(tmpRoot, 159, 0xffffef, 24);
        addChar(tmpRoot, 160, 0x3fffda, 22);
        addChar(tmpRoot, 161, 0x1fffdd, 21);
        addChar(tmpRoot, 162, 0xfffe9, 20);
        addChar(tmpRoot, 163, 0x3fffdb, 22);
        addChar(tmpRoot, 164, 0x3fffdc, 22);
        addChar(tmpRoot, 165, 0x7fffe8, 23);
        addChar(tmpRoot, 166, 0x7fffe9, 23);
        addChar(tmpRoot, 167, 0x1fffde, 21);
        addChar(tmpRoot, 168, 0x7fffea, 23);
        addChar(tmpRoot, 169, 0x3fffdd, 22);
        addChar(tmpRoot, 170, 0x3fffde, 22);
        addChar(tmpRoot, 171, 0xfffff0, 24);
        addChar(tmpRoot, 172, 0x1fffdf, 21);
        addChar(tmpRoot, 173, 0x3fffdf, 22);
        addChar(tmpRoot, 174, 0x7fffeb, 23);
        addChar(tmpRoot, 175, 0x7fffec, 23);
        addChar(tmpRoot, 176, 0x1fffe0, 21);
        addChar(tmpRoot, 177, 0x1fffe1, 21);
        addChar(tmpRoot, 178, 0x3fffe0, 22);
        addChar(tmpRoot, 179, 0x1fffe2, 21);
        addChar(tmpRoot, 180, 0x7fffed, 23);
        addChar(tmpRoot, 181, 0x3fffe1, 22);
        addChar(tmpRoot, 182, 0x7fffee, 23);
        addChar(tmpRoot, 183, 0x7fffef, 23);
        addChar(tmpRoot, 184, 0xfffea, 20);
        addChar(tmpRoot, 185, 0x3fffe2, 22);
        addChar(tmpRoot, 186, 0x3fffe3, 22);
        addChar(tmpRoot, 187, 0x3fffe4, 22);
        addChar(tmpRoot, 188, 0x7ffff0, 23);
        addChar(tmpRoot, 189, 0x3fffe5, 22);
        addChar(tmpRoot, 190, 0x3fffe6, 22);
        addChar(tmpRoot, 191, 0x7ffff1, 23);
        addChar(tmpRoot, 192, 0x3ffffe0, 26);
        addChar(tmpRoot, 193, 0x3ffffe1, 26);
        addChar(tmpRoot, 194, 0xfffeb, 20);
        addChar(tmpRoot, 195, 0x7fff1, 19);
        addChar(tmpRoot, 196, 0x3fffe7, 22);
        addChar(tmpRoot, 197, 0x7ffff2, 23);
        addChar(tmpRoot, 198, 0x3fffe8, 22);
        addChar(tmpRoot, 199, 0x1ffffec, 25);
        addChar(tmpRoot, 200, 0x3ffffe2, 26);
        addChar(tmpRoot, 201, 0x3ffffe3, 26);
        addChar(tmpRoot, 202, 0x3ffffe4, 26);
        addChar(tmpRoot, 203, 0x7ffffde, 27);
        addChar(tmpRoot, 204, 0x7ffffdf, 27);
        addChar(tmpRoot, 205, 0x3ffffe5, 26);
        addChar(tmpRoot, 206, 0xfffff1, 24);
        addChar(tmpRoot, 207, 0x1ffffed, 25);
        addChar(tmpRoot, 208, 0x7fff2, 19);
        addChar(tmpRoot, 209, 0x1fffe3, 21);
        addChar(tmpRoot, 210, 0x3ffffe6, 26);
        addChar(tmpRoot, 211, 0x7ffffe0, 27);
        addChar(tmpRoot, 212, 0x7ffffe1, 27);
        addChar(tmpRoot, 213, 0x3ffffe7, 26);
        addChar(tmpRoot, 214, 0x7ffffe2, 27);
        addChar(tmpRoot, 215, 0xfffff2, 24);
        addChar(tmpRoot, 216, 0x1fffe4, 21);
        addChar(tmpRoot, 217, 0x1fffe5, 21);
        addChar(tmpRoot, 218, 0x3ffffe8, 26);
        addChar(tmpRoot, 219, 0x3ffffe9, 26);
        addChar(tmpRoot, 220, 0xffffffd, 28);
        addChar(tmpRoot, 221, 0x7ffffe3, 27);
        addChar(tmpRoot, 222, 0x7ffffe4, 27);
        addChar(tmpRoot, 223, 0x7ffffe5, 27);
        addChar(tmpRoot, 224, 0xfffec, 20);
        addChar(tmpRoot, 225, 0xfffff3, 24);
        addChar(tmpRoot, 226, 0xfffed, 20);
        addChar(tmpRoot, 227, 0x1fffe6, 21);
        addChar(tmpRoot, 228, 0x3fffe9, 22);
        addChar(tmpRoot, 229, 0x1fffe7, 21);
        addChar(tmpRoot, 230, 0x1fffe8, 21);
        addChar(tmpRoot, 231, 0x7ffff3, 23);
        addChar(tmpRoot, 232, 0x3fffea, 22);
        addChar(tmpRoot, 233, 0x3fffeb, 22);
        addChar(tmpRoot, 234, 0x1ffffee, 25);
        addChar(tmpRoot, 235, 0x1ffffef, 25);
        addChar(tmpRoot, 236, 0xfffff4, 24);
        addChar(tmpRoot, 237, 0xfffff5, 24);
        addChar(tmpRoot, 238, 0x3ffffea, 26);
        addChar(tmpRoot, 239, 0x7ffff4, 23);
        addChar(tmpRoot, 240, 0x3ffffeb, 26);
        addChar(tmpRoot, 241, 0x7ffffe6, 27);
        addChar(tmpRoot, 242, 0x3ffffec, 26);
        addChar(tmpRoot, 243, 0x3ffffed, 26);
        addChar(tmpRoot, 244, 0x7ffffe7, 27);
        addChar(tmpRoot, 245, 0x7ffffe8, 27);
        addChar(tmpRoot, 246, 0x7ffffe9, 27);
        addChar(tmpRoot, 247, 0x7ffffea, 27);
        addChar(tmpRoot, 248, 0x7ffffeb, 27);
        addChar(tmpRoot, 249, 0xffffffe, 28);
        addChar(tmpRoot, 250, 0x7ffffec, 27);
        addChar(tmpRoot, 251, 0x7ffffed, 27);
        addChar(tmpRoot, 252, 0x7ffffee, 27);
        addChar(tmpRoot, 253, 0x7ffffef, 27);
        addChar(tmpRoot, 254, 0x7fffff0, 27);
        addChar(tmpRoot, 255, 0x3ffffee, 26);
        addEOS(tmpRoot, 256, EOS_LSB, EOS_LENGTH);

        // The difference in performance can always be checked by not using
        // the immutable trie:
        //     return tmpRoot;
        return ImmutableNode.copyOf(tmpRoot);
    }

    private QuickHuffman() {
    }

    private static void addChar(Node root, int symbol, int code, int bitLength) {
        addLeaf(root, (char) symbol, code, bitLength, false);
        long value = ((long) code) << (64 - bitLength); // re-align MSB <- LSB
        codes[symbol] = value | bitLength;
    }

    private static void addEOS(Node root, int symbol, int code, int bitLength) {
        addLeaf(root, (char) symbol, code, bitLength, true);
    }

    private static void addLeaf(Node root,
                                char symbol,
                                int code,
                                int bitLength,
                                boolean isEOS) {
        assert 0 < bitLength && bitLength <= 32 : bitLength;
        Node curr = root;
        int nBytes = bytesForBits(bitLength);
        // The number of bits the code needs to be shifted to the left in order
        // to align with the byte #nBytes boundary:
        int align = (nBytes << 3) - bitLength;
        code <<= align;
        // descend the trie until the last element
        int l = 0;
        for (int i = 0, probe = 0xff << ((nBytes - 1) << 3);
             i < nBytes - 1;
             i++, probe >>>= 8) {
            curr.setEOSPath(curr.isEOSPath() | isEOS);
            int idx = (code & probe) >>> ((nBytes - 1 - i) << 3);
            curr = curr.getOrCreateChild(idx);
            curr.setLength(8);
            l += 8;
        }
        // Assign the same char to all byte variants. For example, if the code
        // and its length are 00011b and 5 respectively (letter 'a') then, in
        // order to be able to match any byte starting with 00011b prefix,
        // the following nodes need to be created:
        //
        //     00011000b, 00011001b, 00011010b, 00011011b, 00011100b, 00011101b,
        //     00011110b and 00011111b
        int idx = code & 0xff;
        curr.setEOSPath(curr.isEOSPath() | isEOS);
        for (int i = 0; i < (1 << align); i++) {
            Node child = curr.getOrCreateChild(idx | i);
            child.setSymbol(symbol);
            child.setEOSPath(child.isEOSPath() | isEOS);
            child.setLength(bitLength - l);
        }
    }

    /*
     * A node in the Huffman trie.
     */
    interface Node {

        boolean isEOSPath();

        void setEOSPath(boolean value);

        boolean isLeaf();

        Node getChild(int index);

        Node getOrCreateChild(int index);

        Node[] getChildren();

        char getSymbol();

        void setSymbol(char symbol);

        int getLength();

        void setLength(int value);
    }

    /*
     * Mutable nodes used for initial construction of the Huffman trie.
     * (These nodes are perfectly ok to be used after that.)
     */
    static final class TemporaryNode implements Node {

        private char symbol;
        private boolean eosPath;
        private TemporaryNode[] children;
        private int length;

        @Override
        public TemporaryNode getOrCreateChild(int index) {
            ensureChildrenExist();
            if (children[index] == null) {
                children[index] = new TemporaryNode();
            }
            return children[index];
        }

        private void ensureChildrenExist() {
            if (children == null) {
                children = new TemporaryNode[256];
            }
        }

        @Override
        public boolean isLeaf() {
            return children == null;
        }

        @Override
        public boolean isEOSPath() {
            return eosPath;
        }

        @Override
        public void setEOSPath(boolean value) {
            eosPath = value;
        }

        @Override
        public TemporaryNode getChild(int index) {
            ensureChildrenExist();
            return children[index];
        }

        @Override
        public Node[] getChildren() {
            if (children == null) {
                return new Node[0];
            }
            return children;
        }

        @Override
        public char getSymbol() {
            return symbol;
        }

        @Override
        public int getLength() {
            return length;
        }

        @Override
        public void setSymbol(char value) {
            this.symbol = value;
        }

        @Override
        public void setLength(int value) {
            this.length = value;
        }
    }

    /*
     * Immutable node used to construct traversal-only Huffman trie.
     *
     * Once the trie has been built, the support of modifications is no longer
     * required. An immutable trie should be used. Not only it will help to
     * catch possible bugs, but hopefully speedup the traversal operations.
     */
    static final class ImmutableNode implements Node {

        private final char symbol;
        private final boolean eosPath;
        private final int length;
        private final List<ImmutableNode> children;

        public static ImmutableNode copyOf(Node node) {
            if (node.isLeaf()) {
                return new ImmutableNode(node.getSymbol(), node.isEOSPath(),
                        node.getLength());
            }
            Node[] children = node.getChildren();
            ImmutableNode[] immutableChildren = new ImmutableNode[children.length];
            for (int i = 0; i < children.length; i++) {
                immutableChildren[i] = copyOf(children[i]);
            }
            return new ImmutableNode(node.isEOSPath(), node.getLength(),
                    immutableChildren);
        }

        /* Creates a leaf node */
        private ImmutableNode(char symbol,
                              boolean eosPath,
                              int length) {
            this.symbol = symbol;
            this.eosPath = eosPath;
            this.length = length;
            this.children = Collections.emptyList();
        }

        /* Creates a node with children */
        private ImmutableNode(boolean eosPath,
                              int length,
                              ImmutableNode[] children) {
            this.symbol = 0;
            this.eosPath = eosPath;
            this.length = length;
            if (children.length == 0) {
                throw new IllegalArgumentException();
            }
            // A list produced by List.of should not be slower than array for
            // accessing elements by index, and hopefully use additional
            // optimizations (e.g. jdk.internal.vm.annotation.Stable)
            this.children = Arrays.asList(children);
        }

        @Override
        public boolean isLeaf() {
            return children.isEmpty();
        }

        @Override
        public boolean isEOSPath() {
            return eosPath;
        }

        @Override
        public void setEOSPath(boolean value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ImmutableNode getChild(int index) {
            return children.get(index);
        }

        @Override
        public ImmutableNode getOrCreateChild(int index) {
            throw new UnsupportedOperationException();
        }

        @Override
        public ImmutableNode[] getChildren() {
            // This method is not expected to be called on an immutable node.
            // If it is called, it requires some investigation.
            throw new UnsupportedOperationException();
        }

        @Override
        public char getSymbol() {
            return symbol;
        }

        @Override
        public void setSymbol(char symbol) {
            throw new UnsupportedOperationException();
        }

        @Override
        public int getLength() {
            return length;
        }

        @Override
        public void setLength(int value) {
            throw new UnsupportedOperationException();
        }
    }

    static final class Reader implements Huffman.Reader {

        private final HPACK.BufferUpdateConsumer UPDATER =
                (buf, bufLen) -> {
                    buffer = buf;
                    bufferLen = bufLen;
                };

        private Node curr = root;  // current position in the trie
        private long buffer;       // bits left from the previous match (aligned to the left, or MSB)
        private int bufferLen;     // number of bits in the buffer
        private int len;           // length (in bits) of path to curr
        private boolean done;

        @Override
        public void read(ByteBuffer source,
                         Appendable destination,
                         boolean isLast) throws IOException {
            while (!done) {
                int remaining = source.remaining();
                int nBytes = HPACK.read(source, buffer, bufferLen, UPDATER);
                // write as much as possible
                while (true) {
                    if (bufferLen < 8) {
                        if (nBytes < remaining) { // read again
                            break;
                        } else if (!isLast) { // exit the method to accept more input
                            return;
                        } else if (bufferLen > 0) { // no more data is expected, pad
                            // (this padding may be done more than once)
                            buffer |= ((0xff00000000000000L >>> bufferLen)
                                    & 0xff00000000000000L);
                            // do not update bufferLen, since all those ones are
                            // synthetic and are appended merely for the sake of
                            // lookup
                        } else {
                            done = true;
                            break;
                        }
                    }
                    int idx = (int) (buffer >>> 56);
                    Node node = curr.getChild(idx);
                    if (node == null) { // TODO: TEST
                        throw new IOException("Unexpected byte");
                    }
                    if (node.isLeaf()) {
                        if (node.getLength() > bufferLen) { // matched more than we actually could (because of padding)
                            throw new IOException(
                                    "Not a EOS prefix padding or unexpected end of data");
                        }
                        if (node.isEOSPath()) {
                            throw new IOException("Encountered EOS");
                        }
                        destination.append(node.getSymbol());
                        curr = root;
                        len = 0;
                    } else {
                        curr = node;
                        // because of the padding, we can't match more bits than
                        // there are currently in the buffer
                        len += Math.min(bufferLen, node.getLength());
                    }
                    buffer <<= node.getLength();
                    bufferLen -= node.getLength();
                }
                if (done && (curr.isEOSPath() && len > 7)) {
                    throw new IOException(
                            "Padding is too long (len=" + len + ") "
                                    + "or unexpected end of data");
                }
            }
        }

        @Override
        public void reset() {
            curr = root;
            len = 0;
            buffer = 0;
            bufferLen = 0;
            done = false;
        }
    }

    static final class Writer implements Huffman.Writer {

        private final HPACK.BufferUpdateConsumer UPDATER =
                (buf, bufLen) -> {
                    buffer = buf;
                    bufferLen = bufLen;
                };

        private CharSequence source;
        private boolean padded;
        private int pos;
        private int end;
        private long buffer;
        private int bufferLen;

        @Override
        public Writer from(CharSequence input, int start, int end) {
//            Objects.checkFromToIndex(start, end, input.length());
            this.pos = start;
            this.end = end;
            this.source = input;
            return this;
        }

        @Override
        public boolean write(ByteBuffer destination) {
            while (true) {
                while (true) { // stuff codes into long
                    if (pos >= end) {
                        break;
                    }
                    char c = source.charAt(pos);
                    if (c > 255) {
                        throw new IllegalArgumentException("char=" + ((int) c));
                    }
                    long len = codeLengthOf(c);
                    if (bufferLen + len <= 64) {
                        buffer |= (codeValueOf(c) >>> bufferLen); // append
                        bufferLen += len;
                        pos++;
                    } else {
                        break;
                    }
                }
                if (bufferLen == 0) {
                    return true;
                }
                if (pos >= end && !padded) { // no more input chars are expected, pad
                    padded = true;
                    // A long shift to 64 will result in the same long, not
                    // necessarily 0L. In which case padding will be performed
                    // incorrectly. If bufferLen is equal to 64, the shift (and
                    // padding) in not performed.
                    // (see https://docs.oracle.com/javase/specs/jls/se10/html/jls-15.html#jls-15.19)
                    if (bufferLen != 64) {
                        buffer |= (EOS_MSB >>> bufferLen);
                        bufferLen = bytesForBits(bufferLen) << 3;
                    }
                }
                int nBytes = HPACK.write(buffer, bufferLen, UPDATER, destination);
                if (nBytes == 0) {
                    return false;
                }
            }
        }

        @Override
        public Writer reset() {
            source = null;
            buffer = 0;
            bufferLen = 0;
            end = 0;
            pos = 0;
            padded = false;
            return this;
        }

        @Override
        public int lengthOf(CharSequence value, int start, int end) {
            int len = 0;
            for (int i = start; i < end; i++) {
                char c = value.charAt(i);
                len += codeLengthOf(c);
            }
            return bytesForBits(len);
        }
    }
}
